Suppressing harmonic signals in ion cyclotron resonance mass spectrometry

ABSTRACT

The invention relates to reducing harmonic signals in FT-ICR spectra. Since harmonic signals in quadrupolar 2ω-detection can be more abundant for the same ion motion in the ICR cell as compared to harmonic signals in classical dipolar 1ω-detection, they could hitherto not be reduced to satisfactory levels by any known method, such as gated deflection during ion introduction into, and correcting for an offset electric field axis in the ICR cell. The present disclosure foresees, in addition to other methods carried out for improving the measurement conditions as the case may be, performing the quadrupolar 2ω-detection at least twice, where the phase of the ion excitation radio frequency is turned by 180° in the second measurement. From the sum transient, a Fourier-transformed spectrum is derived. As a result, the broad band spectra of complex substance mixtures like crude oil become cleaner, and misinterpretations of false (harmonic) peaks are minimized.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for the reduction and elimination ofharmonic signals in ion cyclotron resonance frequency or mass spectra.These undesired harmonic signals may generate peaks, particularly inbroad band spectra of complex mixtures of organic substances, whichcould be erroneously interpreted as true ionic peaks. In multipleelectrode nω-detection where the nν=nω/2π resonance frequency ismeasured as the fundamental frequency, high abundant subharmonic signalsdeteriorate the spectra and complicate their interpretation.Specifically, in quadrupolar 2ω-detection, 1ν-subharmonic signals canappear at undesired high abundance.

Description of the Related Art

Ion cyclotron resonance mass spectrometry (ICR-MS) can be conducted in awide range of operation modes. With extreme narrow band operation,detecting only ion species in a mass (m/z) range around a single Dalton,extremely high mass resolutions on the order of several ten millions(R=m/Δm>10⁷) can be achieved from transients measured up to five minutesor more. Extremely good ultra-high vacuum (p<10⁻⁷ Pascal) is required tominimize the number of collisions between the orbiting ions and residualgas molecules in the cell. The ultra-high mass resolution enablesanalysis of the fine structure of isotope satellites of the molecularion species. This reveals the elemental composition of the ion species,facilitating the determination of a total formula for analytes in thesample under investigation.

On the other hand, broad band operation covering mass ranges of somethousand Daltons offers qualitative and quantitative analysis of complexmixtures of hundreds of organic substances with somewhat reduced massresolution of some millions (R>10⁶), and mass precisions of about onepart per million (1 ppm). Since as many as hundreds or thousands ofindividual ion species may occupy the cyclotron orbit, the usablelengths of image current transients are typically reduced to a fewseconds only. Examples for these complex mixture analyses are crudeoils, oil distillation residues, or mixtures of natural substancesextracted from plants when looking for pharmaceutically interestingingredients or reagents, for instance.

In ICR cells, the axis of the magnetron motion should be theoreticallyidentical with the ICR cell axis, but experience tells that quite oftenthe axis of the ionic magnetron orbit in a cylindrical ICR cell shows aradial offset from the geometric axis of the cell. The offset magnetronorbit adversely influences the cyclotron excitation as well as thedetection process of the ions. It also impairs the detected signal; inthe classical dipolar 1ω-detection it leads to an increase of theintensity of the peaks associated with the even-numbered (e.g. second)harmonics in the Fourier transformed spectrum and further to moreabundant sidebands of the ion signal. In extreme cases, ions can be lostduring the cyclotron excitation, which typically comprises a pulse radiofrequency sweep (so-called “chirp”), when they are on large or largelyoffset magnetron orbits that displace the ions critically close to the(cylinder) mantle electrodes.

If the ICR cell components in ICR-MS are somewhat misaligned, thespectra will become complex by the appearance of harmonic signals oflarger peaks or by peaks representing sidebands from the superpositionof cyclotron and magnetron oscillations. In narrow band ICR, these peaksdo not play a prominent role because they are usually far away from thesignals under investigation. But in broad band ICR mass spectrometry, itwould be helpful to suppress these peaks as much as possible, becausethey tend to appear in ranges of interest of the frequency or massspectrum, and can greatly complicate the interpretation of suchspectrum.

A skilled practitioner will acknowledge that an asymmetry of theelectric field inside the cell can be a consequence of many differentreasons, e.g. a deviation of the individual electrode shapes from theideal ones or a deviation of the complete assembled cell from its idealshape, resulting in different kinds of harmonics. Symmetry errors of theelectric field inside the ICR cell may also arise from asymmetriccontact potentials of connectors from the power supply.

Asymmetric electric fields in the ICR cell can also be a consequence ofcharging-up of individual electrodes. Charging is a general process,which can appear due various reasons, one of which could be a highresistive connection of an electrode to ground. Normally, after everyacquisition cycle, the detection electrodes in the cell should be atground potential. However, if they are connected to ground over a largeresistor, which enables the picking-up of induced image charge signalsof very low amplitude, the swiftness and ease of the discharge afterevery acquisition cycle may be adversely affected. Consequently, theelectrode may maintain its charged state for a while, even after thenext acquisition cycle starts. As a result, an asymmetric electric fieldis induced in the cell due to an electrode being incompletelydischarged. A different type of electrode charging is surface charging.This usually happens if the metallic surface of the electrode carries adielectric layer, which can be polarized or charged and cannot easily bedischarged due to its lack of conductance. Cleaning would be a viablecountermeasure for such contamination.

Document U.S. Pat. No. 8,766,174 B1 (G. Baykut et al.) describes methodsand devices for optimization of electric fields in measurement cells ofFourier transform ion cyclotron resonance mass spectrometers. Thisdocument shall be incorporated herein by reference in its entirety. Theoptimization is based on the rationale that offset and asymmetricelectric fields can appear in ion cyclotron resonance cells andtherefore the axis of the magnetron orbit can become radially displaced.Shifted magnetron orbits negatively affect the cyclotron excitation,deteriorate the FT-ICR signal, increase the intensity of theeven-numbered harmonic signals (in dipolar 1ω-detection), lead tostronger side bands of the FT-ICR signal, and in extreme cases, causeloss of ions. The method helps in probing the shift of the magnetronmotion, detecting parameters indicative of the offset of the electricfield axis and correcting it by trimming it back to the geometric axisof the ICR cell. This results in a decrease or complete elimination ofeven-numbered harmonic peaks, which is most clearly observed by thedecrease of the harmonic peak 2ν₊. A further document (U.S. Pat. No.9,355,830 B2; G. Baykut et al.), also incorporated herein by referencein its entirety, describes how to minimize the magnetron orbit radius bygated deflection during the introduction of ions into the ICR cell.Hereby, the sideband peaks of the FT-ICR signal and its harmonic peaksare reduced. In dipolar 1ω-detection the effect is most clearly observedfor the most abundant sideband peak (2ν₊+ν⁻). But, both methods can alsobe applied to multiple electrode nω-detection.

As stated above, for complex mixtures of substances it is broad bandoperation that is called for, but the short transients reduce the massresolution significantly. To improve broad band mass resolution in spiteof the shortness of the measureable transients, a multiple electrodemeasurement of the cyclotron frequencies may be applied; e.g. aquadrupolar 2ω-detection results in a measurement of the doublefrequency 2ν₊. These 2ω-measurements generate mass spectra with doublethe mass resolution as compared with dipolar 1ω-measurements, butunfortunately also usually produce subharmonic signals with thefrequency 1ν of all genuine mass peaks. A good trimming and tuningaccording to documents U.S. Pat. No. 8,766,174 B1 and U.S. Pat. No.9,355,830 B2 can help to reduce the subharmonic peaks 1ν₊ and theirsideband harmonic peak s (ν₊+ν⁻) to about 1 percent of the measuredfundamental 2ν₊ peak; but there is still a need to further reduce the1ν-subharmonic signals, such as by about another factor of ten, in orderto obtain yet increased evaluability of the acquired transients. Theintensity of the subharmonic peaks (ν₊+ν⁻) is space charge dependent; itmight not be stable after trimming in different applications, or makesthe trimming very difficult due to low intensity signals

SUMMARY OF THE INVENTION

In view of the foregoing, the invention pertains in a first aspect to amethod for reducing 1ν-subharmonic signals in measurements of ICR massspectra by quadrupolar 2ω-detection of transients representing the ionicimage currents in an ICR cell after excitation of ions. The methodcomprises the steps of (dipolar) exciting a first bunch of ions using afirst (start) phase of the excitation wave and measuring a firsttransient by quadrupolar 2ω-detection, (dipolar) exciting a second bunchof ions using a second excitation wave (start) phase differing from thefirst phase by about 180°, and measuring a second transient byquadrupolar 2ω-detection, adding the first and second transients, andtransforming the sum of the first and second transients into a frequencyspectrum (or a mass spectrum or m/z spectrum).

The first aspect of the invention relates to the 2ω-detection of thecyclotron frequency with a quadrupolar arrangement of detectionelectrodes and dipolar arrangement of the excitation electrodes. Theinvention proposes to perform this quadrupolar 2ω-measurement two timeswith two bunches of ions (having substantially the same ioniccomposition), wherein the phase of the excitation wave is turned bysubstantially 180° for the second measurement, and the two transientsare added together. As a result, the 1ν-subharmonic signals are greatlyreduced in the frequency and mass spectra which are obtained by suitabletransformations of the sum of the transients.

The method is favorably applied to a broad band measurement spanning anm/z range of equal to or more than 1000 Dalton up to several thousandsof Daltons.

In various embodiments, the first and second bunches of ions arepreferably derived from a complex substance mixture, such as crude oil,oil distillation residue, or a plant extract.

The first and the second bunches of ions might advantageously comprisesubstantially equal numbers of ions. To achieve this, the ions of thefirst and second bunches of ions can be generated in an ion source whichoperates at substantially constant ionic output, and may be transferredto the ICR cell using a same transfer procedure (including various ionfunnels, ion guides and/or ion traps as the case may be).

In various embodiments, the ion source can be fed with substances from asubstance separator, such as a (liquid) chromatograph or anelectrophoretic device. When doing so, the first and second transientsare preferably measured immediately subsequently in order to guaranteesubstantially the same ionic composition in the different ion bunches.

In an alternative of the method, a first sum-transient may be obtainedby adding measured transients from several bunches of ions usingexcitation at the first phase, and a second sum-transient is obtained ina similar way but using excitation at the second phase, and the firstand second sum-transients can be added to obtain the frequency spectrumby Fourier transforming the overall sum spectrum. In a furthermodification, the first and second sum-transients could be obtained byalternately measuring (at correspondingly alternating excitation wavephases) and adding transients from several bunches of ions.

In various embodiments, also a phase of the detection can be switched byabout 180° between the measurement of the first and second transients,and the 1ν-harmonic signals found therein might be used to preciselydetermine the unperturbed cyclotron frequency ν_(c). If both excitationphase and detection phase are turned by 180° in subsequent transientacquisitions, the signal of the measured fundamental frequency 2ν₊disappears, and the signal of the subharmonic peak 1ν₊ and its side bandpeak (ν₊+ν⁻) remains enhanced. This alternative method can be used toprecisely determine mass values by measuring the side band frequency(ν₊+ν⁻) which is in fact the unperturbed cyclotron frequency ν_(c) inthe ICR cell. This frequency is independent of the electrical (axial)trapping potential of the ICR cell; hence magnetron movement and spacecharge perturbations will not influence the measurements, which canresult in higher accuracy of the mass determination.

In various embodiments, the ICR cell may comprise four quartercylindrical mantle electrodes and two axial trapping electrodes, and theexcitation can include irradiating the ICR cell with a pulse radiofrequency sweep (“chirp”), as is well known to one of skill in the art.

In a second aspect, the invention pertains to a method for measuring ioncyclotron resonance transients that represent the ionic image currentsin an ICR cell, having multiple 2×n mantle electrodes where n>2 is aninteger, after excitation of ions. The method comprises the steps of(dipolar) exciting a first bunch of ions using a first excitation wavephase and measuring a first transient by multiple electrodenω-detection, (dipolar) exciting a second bunch of ions using a secondexcitation wave phase differing from the first phase by about 180°, andmeasuring a second transient by multiple electrode nω-detection, andadding the first and second transients to form a sum transient, whichmay be transformed into a frequency or mass (m/z) spectrum.

The second aspect applies the principles of the invention to multielectrode nω-detection with n>2. So doing will reduce the (n−1, n−3,n−5, . . . )ν-subharmonic signals and the higher frequency (n+1, n+3,n+5, . . . )ν-harmonic signals and their sidebands. But very highabundant signals of the (n−2, n−4, n−6, . . . )ν-subharmonic signals andhigher frequency (n+2, n+4, n+6, . . . )ν-harmonic signals and theirsidebands might still appear. In the case of 3ω-detection, the scheme ofexcitation wave phase switching can be different in that the excitationwave phase is switched by 180° together with the detection phase forsubsequent transient acquisitions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simple form of a cylindrical ICR cell (200) with fourcylinder mantle electrodes (210) to (212) and two end cap (axialtrapping) electrodes (205) and (206).

FIG. 2 exhibits a schematic diagram of the classical operation of anFT-ICR cell using dipolar excitation and dipolar 1ω-detection.

FIG. 3a schematically presents the circuit status of the dipolarexcitation for a quadrupolar 2ω-detection measurement, and FIG. 3b showsthe quadrupolar 2ω-detection.

FIG. 4 shows, in some detail, a simulated frequency spectrum as obtainedfrom a Fourier transformation of a transient simulating a quadrupolar2ω-measurement, without using a subharmonic suppression method accordingto principles of this invention. The simulations were done with acyclotron orbit of 50% of the ICR cell radius, 10% magnetron orbit, and10% offset between magnetron center and axis of the ICR cell(illustrated in the insert at the top right). The main signal representsthe fundamental peak 2ν₊; next in size is the group of the1ν-subharmonics, comprising the cyclotron frequency ν₊ and its side band(ν₊+ν⁻). Furthermore, third, fourth, fifth and sixth harmonics arevisible towards larger frequencies as tiny peaks.

FIG. 5 shows on the left-hand side two excerpts of simulated transientsas measured by quadrupolar 2ω-detection, wherein the measurement at thebottom was obtained with an excitation wave phase turned by 180°compared with the excitation wave phase used for the transient at thetop. The addition of both transients is shown on the right-hand side.

FIG. 6a presents the frequency spectrum, which is identical for each ofthe two transients shown on the left-hand side of FIG. 5, and FIG. 6bshows the frequency spectrum of the summed transient on the right-handside of FIG. 5. The group of 1ν-subharmonic signals completelydisappears (and also the groups of third and fifth harmonic signals).

FIG. 7 again shows on the left-hand side two excerpts of simulatedtransients as measured by quadrupolar 2ω-detection, wherein themeasurement at the bottom was obtained with both excitation anddetection phases turned by 180° compared with the phases of the firstmeasured transient at the top. The addition of both transients is shownon the right-hand side, now showing a kind of beat.

FIG. 8a presents the frequency spectrum, which is identical for each ofthe two transients shown on the left-hand side of FIG. 7, and FIG. 8bshows the frequency spectrum of the summed transient on the right-handside of FIG. 7. Now, the fundamental cyclotron peak 2ν₊ disappears (andalso the group of fourth harmonic signals), while the group of1ν-subharmonics takes full size. This method can be used to preciselymeasure the side band frequency (ν₊+ν⁻) which is in fact the unperturbedcyclotron frequency ν_(c) in the ICR cell. This frequency is notinfluenced by the electrical (axial) trapping potential of the ICR cell,i.e. by magnetron movement and space charge perturbations.

FIG. 9 exhibits in the upper panel a measured (not simulated) broadbandFT-ICR mass spectrum of sodium trifluoroacetate (NaTFA) that mainlyconsists of a series of cluster ion peaks, with the strongest peak atm/z 703 Dalton. In the lower panel, FIG. 9 displays a closer view of thegroup of first subharmonics of this strongest peak, appearing at m/z1405.6 Dalton, magnified in intensity by a factor of 100, and zoomed-inon the m/z scale. The intensity of this group of peaks, not representingtrue ionic signals in the spectrum, amounts to about 1 percent of itsfundamental peak, potentially giving rise to some misinterpretation ofthe spectrum. Similar harmonic peaks may be visible for all other masspeaks of the spectrum.

FIG. 10 demonstrates the effect of the method according to principles ofthe invention. The intensities of the first harmonics of the spectrumpeaks now are reduced by a factor of about ten.

FIG. 11 shows three measurements of a complex mixture sample SRFA(Suwannee River Fulvic Acids). In the uppermost spectrum, the ICR cellwas not correctly trimmed. In the middle spectrum, the cell was almostoptimally trimmed and filled using the methods described in U.S. Pat.No. 8,766,174 B1 and U.S. Pat. No. 9,355,830 B2, respectively. The1ν-subharmonic signals were greatly reduced but did not completelydisappear. In the mass spectrum at the bottom, the excitation phaseswitching according to principles of this invention was usedadditionally to almost completely eliminate the 1ν-subharmonic signals.

FIG. 12 shows by way of example an ICR-MS set-up suitable for carryingout the methods according to the invention.

FIG. 13 shows the three independent ion motions of an ion in an ICRcell. Only the radial motions, the cyclotron motion with frequency ν₊and the magnetron motion with frequency ν⁻ are relevant for carrying outthe methods according to principles of the invention. It should bementioned that the orbit diameters of the radial motions are outlinedschematically here in such a way as to give the impression that thecyclotron motion is a fast motion (small orbit) and the magnetron motionis a slow (drift) motion (large orbit). In fact, the cyclotron frequencyis usually faster by a factor of approximately 10⁵, but the cyclotronorbit is usually larger than the magnetron orbit, contrary to what isshown.

DETAILED DESCRIPTION

While the invention has been shown and described with reference to anumber of different embodiments thereof, it will be recognized by thoseof skill in the art that various changes in form and detail may be madeherein without departing from the scope of the invention as defined bythe appended claims.

In a first aspect, the invention aims to suppress the 1ν-subharmonicsignals in broad band spectra obtained by quadrupolar 2ω-detection. Inspectra of complex mixtures of substances, these signals complicate theinterpretation. In broad band spectra measurement, the measurabletransients are usually short, only a few seconds, reducing theachievable mass resolution. To enhance the resolution by a factor oftwo, quadrupolar 2ω-detection can be applied, but the 1ν-subharmonicsdisturb the spectra.

The first aspect relates to the 2ω-measurement of the cyclotronfrequency, such as conducted with a quadrupolar arrangement of quartercylindrical excitation and detection electrodes, as shown by way ofexample in FIG. 1. The principle of quadrupolar 2ω-measurements isillustrated in FIG. 3a and FIG. 3b , exhibiting the switching states forthe excitation and detection events. To suppress the 1ν-subharmonics,this quadrupolar 2ω-measurement is performed two times, wherein thephase of the excitation wave is turned by 180° for the secondmeasurement, and the two transients are added together. As a result, thesignals of the 1ν-subharmonics are greatly reduced or even eliminatedbeyond detectability.

The result can be studied by computer simulations, an example of whichis presented in FIG. 4. Here, the cyclotron orbit radius is assumed toamount to 50% of the ICR cell radius, and the magnetron orbit radius to10% of the ICR cell radius. The center of the magnetron orbit has anoffset of about 10% from the axis of the ICR cell. The orbit positionsof cyclotron and magnetron are illustrated in the insert at the topright. Under these conditions, a frequency spectrum will be obtained asexhibited in FIG. 4. The designation ν₊ represents the cyclotronfrequency, ν⁻ represents the magnetron frequency. As expected, the mostabundant peak appears at 2ν₊, the double cyclotron frequency; butsurprisingly the 1ν-subharmonics group (ν₊; ν₊+ν⁻) has an intensity ofabout 40% of the main peak. The higher frequency harmonics groups (3ν,4ν, 5ν and 6ν) are visible but have largely negligible intensities.

FIGS. 5, 6 a, and 6 b show the simulation result of the suppressionaccording to principles of this invention. FIG. 5 depicts on theleft-hand side two excerpts of simulated transients as measured by 2ωquadrupolar detection, wherein the measurement at the bottom wasobtained with an excitation phase turned by 180° compared with theexcitation phase used for the transient at the top. The addition of bothtransients, the basic idea of this invention, is shown on the right-handside of the figure. FIG. 6b now presents the frequency spectrum of thesummed transient on the right-hand side of FIG. 5. As intended by theinvention, the 1ν-subharmonics group (ν₊; ν₊+ν⁻) completely disappears,and also the groups of higher frequency third and fifth harmonics. Incontrast, FIG. 6a presents the frequency spectrum of one of theleft-hand side transients of FIG. 5 showing the original spectrum withall subharmonics and higher frequency harmonics.

If both excitation wave phase and detection phase are turned by 180°,the 2ν₊ signal, i.e. the double fundamental frequency, disappears, andthe signal of the 1ν-subharmonics group (ν₊; ν₊+ν⁻) remains, asdemonstrated by FIGS. 7, 8 a, and 8 b. This method can be used toprecisely determine frequency or mass values by measuring the side bandfrequency (ν₊+ν⁻) which is in fact the unperturbed cyclotron frequencyν_(c) in the ICR cell. This frequency is not influenced by theelectrical (axial) trapping potential of the ICR cell, i.e. by magnetronmovement and space charge perturbations.

Real measurements of the effect of the invention are presented in FIG. 9and FIG. 10. In both figures, the upper panel shows measured massspectra of sodium trifluoroacetate (NaTFA), which forms numerous clusterions. In the bottom panels of the figures, the 1ν-subharmonics group ofthe main mass peak of 703 Dalton is shown, appearing around m/z 1405.6,enlarged in intensity by a factor of 100, and zoomed-in on the massscale. In FIG. 9, after application of the shimming and gated deflectionmethods described in U.S. Pat. No. 8,766,174 B1 and U.S. Pat. No.9,355,830 B2, respectively, but without application of the methodpresented herein, the intensities of the 1ν-subharmonics amount to about1% of the corresponding fundamental 2ν₊ peak. In FIG. 10, applying inaddition the principles according to the invention, the signals of the1ν-subharmonics (ν₊; ν₊+ν⁻) are reduced in size by a factor of aboutten.

FIG. 11 shows spectra of measurements of a complex mixture sample SRFA(Suwanee River Fulvic Acids) acquired with three different methods. Inthe uppermost spectrum, the ICR cell was not correctly trimmed byshimming and gated deflection. In the middle spectrum, the cell wasoptimally trimmed according to the methods described in the documentsU.S. Pat. No. 8,766,174 B1 and U.S. Pat. No. 9,355,830 B2 for theoptimization of electric fields and reduction of the magnetron orbit inmeasurement cells of Fourier transform ion cyclotron resonance massspectrometers. The 1ν-subharmonic signals are greatly reduced but do notcompletely disappear. In the mass spectrum at the bottom, the excitation(wave) phase switching according to principles of this invention wasapplied additionally to the aforementioned measures and results in thealmost complete elimination of the 1ν-subharmonic signals.

It should be mentioned here that the method is not restricted to2ω-detection. In a second aspect, it is possible to apply it to multielectrode nω-detection with n>2. Applying the principles disclosedherein will reduce the (n−1, n−3, n−5, . . . )ν-subharmonics and (n+1,n+3, n+5, . . . )ν-harmonics. But with nω-detection, the very highabundant signals of the (n−2, n−4, n−6, . . . )ν-subharmonics andharmonics are still apparent. Also the low abundance signals of (n+2,n+4, n+6, . . . )ν-harmonics remain.

The general operation and function of an ion cyclotron resonance massspectrometer can be briefly described by way of example with referenceto FIG. 12. Ions are produced preferably at substantially constantoutput, for example, by electrospray in a vacuum-external ion source(1). The ion source (1) might receive the liquid to be sprayed from anupstream substance separator (25), such as a liquid chromatograph or anelectrophoretic device. The ions can be introduced, together withambient gas, through a capillary (2) into the first stage (3) of adifferential pumping system, which may consist of a series of chambers(3), (5), (7), (9), (11) and (13) and could be pumped by the pumps (4),(6), (8), (10), (12) and (14). Ions in the chambers (3) and (5) can bedrawn in by the ion funnels (14) and (15) and transferred into themultipole ion guiding system (16), in which ions can be either guidedthrough or also be stored. Storing allows in particular the repeatedgated release of ion bunches having substantially the same ion count.The ions may be subsequently transferred through a quadrupole massfilter (17) and through another multipole ion guide (18) that alsoallows ion storage, and finally via the main ion transfer system (19)into the ICR cell (200), where they can be captured, trapped anddetected.

The ICR cell (200) may consist of four mantle-shaped enclosinglongitudinal electrodes (210) to (212) and of two axial trappingelectrodes (205) and (206) with a central hole (20) in each of them, ashas been set out with reference to FIG. 1. The ICR cell is preferablylocated in the homogeneous zone of a strong magnetic field that may begenerated by superconducting coils in a helium cryostat (24) and shouldbe kept as constant in time as well as spatially homogeneous aspossible. The magnetic field is preferably aligned parallel to thelongitudinal mantle electrodes of the ICR cell, as shown.

The radial motions of an ion in an ICR cell which are relevant forcarrying out the methods according to principles of the invention arethe cyclotron motion with frequency ν₊ and the magnetron motion withfrequency ν⁻ with reference to FIG. 13. The cyclotron motion is a fastmotion perpendicular to the magnetic field lines and the magnetronmotion is a slow (drift) motion around the electric trapping field axis,the cyclotron frequency being typically higher by a factor of about 10⁵.

The invention has been described with reference to a number of differentembodiments thereof. It will be understood, however, that variousaspects or details of the invention may be changed, or various aspectsor details of different embodiments may be arbitrarily combined, ifpracticable, without departing from the scope of the invention.Generally, the foregoing description is for the purpose of illustrationonly, and not for the purpose of limiting the invention which is definedsolely by the appended claims, including any equivalent implementationsas the case may be.

The invention claimed is:
 1. A method for reducing 1ν-subharmonicsignals in measurements of ICR mass spectra by quadrupolar 2ω-detectionof transients representing ionic image currents in an ICR cell afterexcitation of ions, the method comprising the steps: exciting a firstbunch of ions using a first excitation wave phase and measuring a firsttransient by 2ω-detection, exciting a second bunch of ions using asecond excitation wave phase differing from the first phase bysubstantially 180°, and measuring a second transient by 2ω-detection,adding the first and second transients, and transforming a sum of thefirst and second transients into a frequency spectrum.
 2. The methodaccording to claim 1, being applied to a broad band measurement spanningan m/z range of equal to or more than 1000 Dalton.
 3. The methodaccording to claim 2, wherein the first and second bunches of ions arederived from a complex substance mixture.
 4. The method according toclaim 3, wherein the complex substance mixture is derived from one ofcrude oil, oil distillation residue, and a plant extract.
 5. The methodaccording to claim 1, wherein the first and the second bunches of ionscomprise substantially equal numbers of ions.
 6. The method according toclaim 5, wherein the ions of the first and second bunches of ions aregenerated in an ion source which operates at substantially constantionic output, and are transferred to the ICR cell using a same transferprocedure.
 7. The method according to claim 6, wherein the ion source isfed with substances from a substance separator.
 8. The method accordingto claim 7, wherein the substance separator is one of a chromatographand an electrophoretic device.
 9. The method according to claim 7,wherein the first and second transients are measured immediatelysubsequently.
 10. The method according to claim 1, wherein a firstsum-transient is obtained by adding measured transients from severalbunches of ions using the first excitation wave phase, and a secondsum-transient is obtained in a similar way but using the secondexcitation wave phase, and the first and second sum-transients are addedto obtain the frequency spectrum by Fourier transformation.
 11. Themethod according to claim 10, wherein the first and secondsum-transients are obtained by alternately measuring and addingtransients from several bunches of ions.
 12. The method according toclaim 1, wherein also a phase of detection is switched by 180° between ameasurement of the first and second transients, and 1ν signals foundtherein are used to precisely determine an unperturbed cyclotronfrequency ν_(c).
 13. The method according to claim 1, wherein thefrequency spectrum is transformed into a mass spectrum.
 14. The methodaccording to claim 1, wherein the excitation is dipolar.
 15. The methodaccording to claim 1, wherein the ICR cell comprises four quartercylindrical mantle electrodes and two axial trapping electrodes.
 16. Themethod according to claim 1, wherein the excitation includes irradiatingthe ICR cell with a pulse radio frequency sweep (“chirp”).
 17. A methodfor measuring ion cyclotron resonance transients that represent ionicimage currents in an ICR cell, having 2×n mantle electrodes where n>2 isan integer, after excitation of ions, the method comprising the steps:exciting a first bunch of ions using a first excitation wave phase andmeasuring a first transient by nω-detection, exciting a second bunch ofions using a second excitation wave phase differing from the first phaseby substantially 180°, and measuring a second transient by nω-detection,and adding the first and second transients to form a sum transient. 18.The method according to claim 17, wherein the sum transient istransformed into a frequency or mass spectrum.
 19. The method accordingto claim 18, wherein an intensity of a group of (n−1)ν, (n−3)ν, (n−5)ν,. . . subharmonic signals as well as a group of (n+1)ν, (n+3)ν, (n+5)ν,. . . higher frequency harmonic signals is reduced as compared to thatof a main peak nν₊ in the frequency or mass spectrum.
 20. The methodaccording to claim 17, wherein both an excitation wave phase as well asa detection phase is switched by substantially 180° between ameasurement of the first and second transients.